The present invention relates to a method of soldering, which is applied to joining of a silicon blank body to a substrate in a process of manufacturing semiconductor elements such as power transistors and the like.
Recently there is a strong tendency to make small electronic parts such as power transistors and the like, and such electronic parts are often used to the maximum of their rated values. It is therefore necessary to avoid increasing the value of thermal resistance due to faults such as voids or the like in a joint portion. Conventionally the joint between an Si chip and a base substrate has been performed by soldering in a mixture gas of hydrogen and nitrogen. In such a case, a solder is supplied as a form of a foil by inserting between the Si chip and the base substrate.
As a result of X-ray transmission tests for inquiring an Si chip and a substrate as a base joined in the above-mentioned method, it was found that there was a void rate about 10% relative a joint area. In order to reduce such a high void rate, there is a method in which a solder foil is put on at least one side of a surface soldered of the Si chip and a portion of the substrate on which the chip is soldered, and the solder foil is heated without contact each other so as to be melted in a mixture gas of hydrogen and nitrogen to perform preliminary soldering, and thereafter a combination of the Si chip with the substrate is heated to be melted again to perform soldering. However, in such a method of soldering after preparatory soldering, there has been a limit in the void rate reduced to about half of that in the above-mentioned method of soldering. The void rate 5% is usually detected as a fault in thermal resistance, and the fraction defective reaches to 10 to 15%. However, if voids of 5% are scattered all over the area of the soldered surface of an Si chip, such a product may be a non-defective one. If such a product is offered to the market, the deterioration of the soldering portion is accelerated by power or heat cycles, so that its life becomes shorter than that of a normal product to thereby lower the reliability.